This invention relates generally to environmental barrier coatings and, more particularly, to environmental barrier coatings for a component fabricated from a silicon-based substrate material.
Rare earth (RE) disilicate environmental barrier coatings (EBCs), having a general composition of RE2Si2O7, protect gas turbine components fabricated from a silicon-containing ceramic matrix composite (CMC) substrate material or silicon nitride (Si3N4) substrate material from harmful exposure to chemical environments in-service. RE=La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, and Lu, and includes the rare earth-like elements Y and Sc. The rare earth disilicates have coefficients of thermal expansion (CTEs) that are well matched to the CTE of the CMC substrate material. Such disilicates have a minimal tendency to crack in service and during thermal cycling of the component. However, disilicates are vulnerable to leaching of SiO2 and recession by chemical interactions with water vapor in the turbine combustion atmosphere. Such leaching creates a microporous microstructure in the EBC, and an initially dense EBC is converted to a porous layer in less than the required design lifetime. Thus, such disilicates do not have the durability required for the application.
Rare earth (RE) monosilicates, having a general composition of RE2SiO5, have been used as EBCs instead of rare earth disilicates. RE monosilicates have low rates of volatilization in combustion atmospheres containing water vapor and, hence, have low recession rates. However, the monosilicates typically have CTEs that are not well matched to the CTE of the CMC substrate material. As a result, the monosilicate topcoats tend to crack during application, heat treatment and/or service exposure, allowing water vapor to penetrate the topcoat and cause subsurface chemical reactions and/or premature EBC spallation. The extent of such cracking is directly dependent on the thickness of the coating layer and the difference in CTE between the coating layer and the substrate material.
Many conventional EBC materials are deposited on components using a plasma spraying process. The plasma spraying process provides flexibility to deposit a large variety of materials within a wide coating thickness range (ranging from about 0.002 inch to about 0.040 inch) without major process modifications. However, the deposited coating material is often inherently in a thermodynamically metastable state (such as an amorphous phase, a higher temperature phase or one or more non-equilibrium phases) due to rapid quenching during the spray process. Upon exposure to high temperature and transformation to the equilibrium state, the constrained coating can undergo a variety of dimensional changes resulting in stresses in the coating that can lead to various types of cracking behavior. The propensity of the coating to crack tends to be directly proportional to the coating thickness.
For a RE2SiO5 coating processed by plasma spraying, this is found to be particularly problematic, leading to both catastrophic through-thickness cracking and delamination of the coating upon exposure to elevated temperatures. In this case, the through-thickness cracking of the coating material is believed to be mainly driven by the mismatch between the CTE of the coating material, about 6×10−6 1/C to about 7×10−6 1/C, and the CTE of the substrate material, about 4.5×10−6 1/C to about 5.0×10−6 1/C for SiC or a SiC/SiC composite. The delamination of the coating is observed primarily around non-planar regions of the substrate material and/or geometrical discontinuities and surface perturbations. Further, the delamination has been attributed to dimensional changes during the first heating cycle to service temperature. The cracking behavior has been observed for coatings with a thickness of as low as about 0.002 inch. Additionally, the coatings processed by plasma spraying are prone to contain open porosity and/or a network of fine cracks intercepting the otherwise closed pores and voids. For EBC applications, open porosity in the coating can be detrimental. The open porosity provides a path for rapid water vapor penetration and, hence, accelerated localized degradation and/or deterioration of the underlying materials prone to water-vapor mediated oxidation and volatilization.